1. Field of the Invention
The invention relates to an optical position measuring system
2. Description of the Related Art
Optical position measuring systems contain a scanning unit and a scale which is movable relative to the scanning unit in the measuring direction, and on which one grating or several gratings of a periodic grating structure is or are arranged between the scale and the scanning unit for an incremental or absolute measurement of the relative movement between the scale and the scanning unit. The scanning unit includes a part on the transmitting side, having a light source arranged on a board, preferably a light-emitting diode, and an optical collimator device assigned to the light source. A photoelectric detector or photo sensor with different active radiation-sensitive detector areas is arranged on the board in the part of the scanning unit on the receiving side.
The precision and quality of the measurements, the insensitivity to contamination or soiling and mechanical changes of the scale, the structural size, as well as the manufacturing costs of an optical measuring system of the above mentioned type depend to a crucial extent on the type and quality of the optical lens arrangement employed in the scanning unit, which creates an image of the scale grating on the radiation-sensitive periodical detector areas. Therefore the structural size of the scanning unit primarily determines the size of the optical position measuring system. The cost of manufacturing is determined to a large extent by the costs of the photoelectric detector, which can be produced in a cost-effective manner if it can be employed in connection with different measuring methods.
The so-called “transmitted light measuring method”, for one, is employed as the measuring method for optical position measurements, wherein the light source, the optical lens arrangement and the scanning plate are arranged on one side, and the photoelectric detector on the other side of the scale and, on the other hand, the so-called “incident light measuring method” is used, wherein the light source, the optical lens arrangement, the scanning plate and the photoelectric detector are arranged on one side of the scale. Making a photoelectric detector available which could be suitable for both methods would allow the production of large numbers and therefore low production costs. However, this requires a special intervention in the path of the scanning beams because, different from the transmitted light measuring method, where the photoelectric detector can be arranged directly next to the scale for detecting the periodic grating structure, with the incident light measurement method the photoelectric detector is arranged farther away from the scale, so that a special optical imaging device is required for creating a sharp image of the scale grating on the periodic radiation-sensitive detector areas for imaging the periodic scale grating on the photoelectric detector with a defined image magnification factor.
For example, the required optical imaging device can be realized with a single lens which, however, because of the required large image field, is of a correspondingly large size and cannot be considered for as compact as possible a scanning unit.
An optical imaging device in the scanning unit of an optical position measuring system is known from GB 2 056 660, which is embodied in the manner of a lens array. The optical imaging device contains a light-emitting diode arranged in a first chamber, from which light beams are emitted through a condenser lens toward the scale, and are reflected by the latter by an optical lens device also embodied as a condenser lens to a light sensor provided in a second chamber arranged next to the first chamber. In principle it is possible to embody a total system of smaller size by an optical imaging device designed as a lens array.
However, GB 2 056 660 does not contain any suggestions as to how such a lens array is to be actually designed as an optical image device in the scanning beam path of an optical position measuring system, in particular, if a defined image magnification factor has been preset, by which a periodic scale grating, for example, is to be imaged on a periodic detector arrangement.